Smectitic soils have high prospect to be developed foragricultural land under a proper soil and plant management. Thesoils are commonly high in total K content. It’s availability forplant growth, however, is relatively low due to fixation bysmectite in interlayer space. Researches aimed to study theeffect of oxalic acid, Na

+

, NH

4+

, and Fe

3+

on availability of soilK, plant N, P, and K uptake, as well as maize yield in smectiticsoils have been conducted in Laboratory of Research and SoilTest and Green House of Indonesian Soil Research Institute,Bogor. Four different types of bulk soil samples taken from Bogor(Typic Hapludalfs), Cilacap (Chromic Endoaquerts), Ngawi (TypicEndoaquerts), and Blora (Typic Haplustalfs) were used forexperiments. Incubation and pot experiments were set up usingFactorial Randomized Completely Block Design with threereplication and pot experiment used maize of Pioneer 21 varietyas plant indicator. The first factor was oxalic acid rates: 0;1,000; 2,000; and 4,000 ppm, while the second one wasapplication of cations: without cation, Na

+

, NH

4+

, and Fe

3+

fromNaCl, NH

4

Cl, and FeCl

3

respectively with 50% of maximumadsorption rate. The Fe

3+

with 50% of maximum adsorption ratecaused plant death, thus the experiment was repeated in the nextseason with Fe

significantly increased the availability of soil K in both Alfisolsand Vertisols where the effect was higher in Vertisols thanAlfisols. The effectiveness of the treatments to release K fromnon available to available K form in the soils was in order of Fe